Metal surface treatment



Full

METAL sunraca TREATMENT Lowell D. Eubank, Richland, Wasln, assignor tothe United States of America as represented by the United States AtomicEnergy Commission No Drawing. Application March 16, N45 Serial No.583,176

8 Claims. (Cl. 117-51) This invention relates to the application ofmetal coatings to uranium. It is especially concerned with the hotdipping process in accordance with which such coatings are applied bydipping the article to be coated into a molten metal bath of the coatingmetal.

The application of metal coatings to uranium by dip-- ping the uraniumin molten metal baths is hindered by the rapidity with which uraniumforms an oxide film and by the difliculty of completely eliminating thisfilm once it has formed. While the surface may be improved by variouspickling treatments, oxide forms on the metal while it is beingtransferred from the pickling bath to the coating bath. The conventionalfluxing materials used in the treatment of iron and steel and variousother metals react violently with uranium and the reaction productsprevent wetting of the metal by the coating bath. As a consequence, themetal coatings which have been applied by the hot dipping method havenot been uniformly adherent.

It is an object of the invention to prepare metallic uranium surfacesfor the application of coatings by the hot dipping process. Furtherobjects are to improve the adherence of the coating metal and to assistthe flow of coating metal over the uranium surface so as to provideuniform and firmly adherent coatings. Further objects are theelimination of oxide film from metallic uranium to be coated and theprevention of oxide formation upon the surface of the molten metalcoating bath. Further objects will appear from the followingdescription.

In accordance with the present invention a molten alkali-metal halide ofthe group consisting of alkali-metal chlorides and bromides is used as aflux for the application of metal coatings to uranium.

In addition to the alkali-metal chloride or bromide, other alkali-metalhalides or alkaline earth metal halides may be present. Thus thefluorides and iodides of the alkali-metals and alkaline earth metals aswell as the chloridesrand bromides of the latter metals may be present.By varying the proportions of the salts, fluxes of melting points(minimum temperatures at which solids are absent) varying as desiredfrom about 320 to 800 C. may be prepared. The alkali-metal halide fluxeshave been found to be applicable to the coating of uranium with metalsby the hot dipping method in general. Examples of such coating metalsare zinc, aluminum, and alloys such as solder, bronze, brass, andaluminum-silicon.

Lithium chloride contributes to alkali-metal halide fluxes verydesirable properties of increased activity and reduced melting point.The greater activity makes possible rapid and complete coverage ofuranium with coating metal. The reduced melting point makes possible theuse of alkali-metal chloride fluxes at lower coating temperatures andhence under a wider variety of coating conditions. This property isespecially important in the application of low-melting coating metals,such as Zinc.

The coating may be effected in conventional dipping apparatusesemploying an appropriately heated vessel for mately the proportions ofthe triple eutectic are highly satisfactory.

Sodium and potassium chlorides and bromides become more effective as themetal bath temperature is increased, and consequently may be usedsatisfactorily without lithium for the application of high-meltingcoating metals.

Mixtures of the chlorides of sodium, potassium, and barium, liquid atabout 550 C. have been used satisfactorily for the application of copperalloy coatings. Since these fluxes do not attack graphite orsiliconcarbide-boncled graphite, for instance, Tercod, they have anadvantage over lithium-chloride-containing fluxes in that vessels ofsuch materials can be used to contain a flux-covered metal without thenecessity of using a liner of porcelain or other especiallyflux-resistant material.

Mixtures of the chlorides of calcium, barium, and sodiurn, liquid below600 C. have been used satisfactorily for the application of aluminumalloy coatings.

In addition to the advantages enumerated above, the alkali-metal halidefluxes have the ability to maintain the content of uranium in moltenmetal dipping baths at a low value. This advantage has been particularlynoticeable in the application of bronze coatings such as copper-tineutectic and speculum metal.

In the application of coatings from baths containing aluminum as theprincipal component, for example, aluminum and aluminum-silicon eutecticmixture, alkalimetal halide fluxes containing a fluoride render theapplication easier than fluxes of chlorides alone. Thus the addition of5% to 15% of sodium fluoride to a triple eutectic alkali-metal chloridemixture or to a calcium, barium, sodium chloride mixture has provedespecially beneficial.

The following examples, in which quantities are expressed as parts byweight, further illustrate the invention:

Example 1 A solid metallic uranium rod 1.1 inches in diameter and 4inches in length" was cleaned by dipping for 20 seconds in aqueous 50%HNO solution containing 1% HQ and was then coated with zinc by dippingit through a molten flux comprising 30% lithium chloride, 50% potassiumchloride and 20% sodium chloride into a molten zinc (Horsehead Special,99.99-l-percent zinc) bath at a temperature of 532 C. The rod was heldin the molten metal for one minute during which the temperature of themetal dropped to 522 C. The rod was removed from the bath, shakenbriskly to remove excess metal, and promptly placed in the trough formedby a pair of smooth asbestos cement (Transite) rollers, each 5 /2 inchesin diameter, and spaced inch from each other and rotated at a speed of130440 R. P. M.

The coated metal rod was rotated in this manner until the zinc coatinghad solidified. It was then cooled to normal temperature by quenching inwater. The resulting coating was smooth and uniform throughout thelength of the rod and over both ends of the rod.

Example 2 A metal rod, the surface of which had been machined,

was pickled for three minutes in 50% nitric acid at about 70 C., rinsedin water and dried. It was then dipped into a molten metal bathprotected by a molten alkalimetal halide flux in such a manner that therod was completely contacted with the flux as it passed into the metalbath and as it was withdrawn from the metal bath. The metal bath,consisting of 94% zinc and 6% aluminum, was at a temperature of 490 C.The protective flux consisted of a mixture of 42 parts of lithiumchloride, 53 parts of potassium chloride and parts of sodium chloride.The period of contact of the uranium rod with the metal bath was fourminutes.

Upon withdrawal from the bath, the rod was rotated on smooth asbestoscement rollers for two minutes to cause the coating to cool andsolidify. It was then placed in an annealing oven maintained at atemperature of about 250 C. for three hours and was then further cooledin air to room temperature in about 40 minutes.

The coating was smooth, uniform and free from pinholes. The coating wastested by maintaining it in air heated to 200 C. It was found to be freeof defects after 70 days of such exposure.

Example 3 A uranium rod was pickled for 2 minutes at 60 C. in aqueous50% nitric acid solution, rinsed, and wiped dry. It was then passedthrough a molten protective flux into a molten metal bath at 600 C. andmaintained in the bath for 2 /2 minutes. The molten bath consisted of85% zinc and 15% aluminm. The protective flux had the same compositionas that of Example 2.

After the rod had remained in the molten metal for 2 /2 minutes, it waswithdrawn through the flux, rolled for 2 /2 minutes on smooth asbestoscement rollers in air to cool and solidify the coating and then annealedat 250 C. for 2 /2 hours, after which it was cooled in an.

The coating was dull, smooth and free from defects. When tested byheating in air at 200 C., it was found to be unimpaired at the end of 70days.

Example 4 A clean metallic uranium rod was dipped through a moltenprotective flux of the composition 56% potassium chloride and 44% sodiumchloride into a molten metal bath comprising 67 parts of copper and 33parts of tin maintained at a temperature of about 815 C. The rod wasallowed to remain in the metal bath for about 1 minute and then waswithdrawn through the protective flux, centrifuged for seconds to removesurplus coating metal, and allowed to cool by exposure to air.

A uniform coating of bronze (speculum metal) on the uranium was thusobtained.

Example 5 A clean metallic uranium rod was dipped for seconds into amolten alkali-metal halide flux comprising 42% sodium chloride, 6%potassium chloride, and 52% calcium chloride on a molten metal bathcomprising 82 parts of tin and 18 parts of nickel at a temperature ofabout 950 C. and then into the molten metal for 60 seconds. The rod waswithdrawn from the bath through the flux and cooled in air.

The rod was completely coated with the tin-nickel alloy.

Example 6 A metallic uranium rod, after a 10 minute cleaning in 50% HNOat about 70 C. and a water rinse, was preheated for 5 minutes in amolten flux consisting of 58 parts of lithium chloride and 42 parts ofpotassium chloride and then passed directly into an underlying molten.metal bath consisting of about 52 /2% tin, 46 /2% copper and 1% nickel,maintained at a temperature of 720 C. The uranium rod was maintained inthis metal bath CPI for 30 seconds, withdrawn through the molten fluxand cooled in air.

The rod was completely and uniformly coated with the bronze coatingalloy.

Example 7 A metallic uranium rod was cleaned in 50% HNO at roomtemperature, rinsed with water and then dipped through a flux comprising37% LiCl, 53% KCl, and 10% NaCl into a molten zinc bath (99.99+% Zn) at433 C. The rod was coated completely with an adherent coating of zincabout 3 mils thick.

Example 8 A metallic uranium rod inch in diameter and 2 inches in lengthwas dipped in aqueous 50% H'NO solution at about 65 C. for 3 minutes. Itwas then dipped through a molten flux of the composition 37% LiCl, 53%KCl and 10% NaCl into a molten metal bath containing 94 parts of zinc(-Prime \Vestern Spelter) and 6 parts of aluminum at 485 C.

The rod was withdrawn from the coating bath and rolled in air on carbonrollers for 2 minutes to allow the coating to solidify. It was thendipped in cold water to cool the rod to normal temperature.

The rod was then placed in an oven through which air at 200 C. wascirculated, to determine its resistance to corrosion. After 135 days ofthis treatment, the rod was still in perfect condition.

Example 9 A metallic uranium rod cleaned by immersing for five minutesin aqueous 50% PIN-O at 65 C. was clipped through a molten flux of about565 C. melting point, prepared by adding to parts of flux of 53% KCl,42% LiCl and 5% NaCl, 10 parts of NaF,'and into a molten metal bathcomprising 88% aluminum and 12% silicon at 620 C. After 3 minutes in thealuminum-silicon alloy bath the rod was withdrawn and rolled on cold,smooth steel rolls until the coating solidified. The rod was completelycoated with a smooth, even coating of the aluminum alloy.

Example 10 A uranium rod was pickled for five minutes in 50% nitric acidsolution at 6070 C., rinsed and dried. It was then passed through aprotective fiux layer consisting of 48% barium chloride, 31% potassiumchloride and 21% sodium chloride into a molten metal bath held at705-715" C. and was maintained in the bath for 40 to 50 seconds. Themolten metal bath was composed of 53 parts tin and 47 parts copper.After 40 to 50 seconds in the bath the rod was withdrawn and quenched inwater. A complete coating of bronze was obtained.

Example 11 A uranium rod was pickled for five minutes in 50% nitric acidsolution at 6070 C., rinsed and dried. The rod was then passed through aflux layer consisting of 45% calcium chloride, 30% barium chloride, 20%sodium chloride and 5% sodium fluoride into a molten metal bath at 600C. and maintained in the bath for 30 seconds. The molten metal bathconsisted of 88 parts aluminum and 12 parts silicon. After 30 secondsthe rod was removed from the molten metal bath and quenched in water. Acomplete coating of aluminumsilicon alloy was obtained.

It will be understood that -I intend to include variations andmodifications of the invention and that the preceding examples areillustrations only and in no wise to be construed as limitations uponthe invention, the scope of which is defined in the appended claims,wherein I claim:

1. In the application of a metal coating to uranium by dipping in amolten metal bath, the improvement which comprises passing the uraniumarticle to be coated through a molten alkali-metal halide protectiveflux into the molten metal bath, said flux consisting of lithiumchloride.

2. In the application of a low-melting metal coating to uranium bydipping in a bath of the molten coating metal, the improvement whichcomprises passing the uranium article to be coated through a moltenalkalimetal halide protective flux into the molten metal bath, said fluxconsisting of a mixture of lithium and potassium chlorides having amelting point between 350 and 400 C.

3. In the application of a metal coating to uranium by dipping in amolten metal bath, the improvement which comprises passing the uraniumarticle to be coated through a molten alkali-metal halide protectiveflux into the molten metal bath, said flux consisting of potassium,lithium, and sodium chlorides in approximately the proportions of thetriple eutectic.

4. In the application of a metal coating to uranium by dipping in amolten metal bath, the improvement which comprises passing the uraniumarticle to be coated through a molten alkali-metal halide protectiveflux into the molten metal bath, said flux consisting of at least onealkali-metal chloride and up to of alkali-metal fluoride.

5. In the application of a metal coating to uranium by dipping in amolten metal bath containing aluminum as its principal component, theimprovement which comprises passing the uranium article to be coatedthrough a molten alkali-metal halide protective flux into the moltenmetal bath, said flux consisting of at least one alkali-metal chlorideand up to 10% of alkali-metal fluoride.

6. A process for coating uranium by immersion into a molten metal bath,comprising passing the uranium article to be coated through an alkalimetal halide-containing flux selected from the group consisting ofalkali metal chloride, a mixture of alkali metal chloride and alkalimetal fluoride, and a mixture of alkali metal chloride and alkalineearth metal chloride.

7. In the application of a metal coating to uranium by dipping in amolten metal bath, the improvement which comprises passing the uraniumarticle to be coated into said molten metal bath through a molten fluxconsisting of about 48% barium chloride, 31% potassium chloride, and 21%sodium chloride.

8. In the application of a metal coating to uranium by dipping in amolten metal bath, the improvement which comprises passing the uraniumarticle to be coated into said molten metal bath through a molten fluxconsisting of alkali metal halide at least of which is alkali metalchloride.

References Cited in the file of this patent UNITED STATES PATENTS611,922 Midgely et al. Oct. 4, 1898 1,114,792 Monnot Oct. 27, 19141,261,110 Fahrenwald Apr. 2, 1918 1,467,398 Schumacher et a1. Sept. 11,1923 1,573,083 Marden et al. Feb. 16, 1926 1,597,189 Gero Aug. 24, 19261,941,750 Johansson Jan. 2, 1934 2,299,166 Miller Oct. 20, 19422,315,725 Moller Apr. 6, 1943

6. A PROCESS FOR COATING URANIUM BY IMMERSION INTO A MOLTEN METAL BATH,COMPRISING PASSING THE URANIUM ARTICLE TO BE COATED THROUGH AN ALKALIMETAL HALIDE-CONTAINING FLUX SELECTED FROM THE GROUP CONSISTING OFALKALI METAL CHLORIDE, A MIXTURE OF ALKALI METAL CHLORIDE AND ALKALIMETAL FLUORIDE, AND A MIXTURE OF ALKALI METAL CHLORIDE AND ALKALINEEARTH METAL CHLORIDE.